Characterisation of the lowest singlet and triplet excited states of S-flurbiprofen

Transient UV-vis absorption spectroscopic characterisation of 2'-methoxyacetophenone as a DNA photosensitiser

2'-Methoxyacetophenone (2M) presents improved UVA absorption as compared with other acetophenone derivatives. On the basis of transient infrared spectroscopy it has been previously claimed that 2M is an interesting photosensitiser for cyclobutane pyrimidine dimers (CPDs) formation. In the present paper, a complete UV-Vis transient absorption spectroscopic characterisation of this compound is provided, including triplet-triplet spectra, triplet lifetimes and rate constants for quenching of 2M by a dimeric thymine derivative. Furthermore, generation of singlet oxygen has been proven by time-resolved near IR phosphorescence measurements. Overall, the obtained results confirm the potential of 2M as a DNA photosensitiser, not only for CPDs formation, but also for oxidative damage.

Competition between "Meta Effect" Photochemical Reactions of Selected Benzophenone Compounds Having Two Different Substituents at Meta Positions

Recent studies conducted on some "meta effect" photochemical reactions focused on aromatic carbonyls having a substitution on one meta position of the benzophenone (BP) and anthraquinone parent compound. In this paper, two different substitutions were introduced with one at each meta position of the BP parent compound to investigate possible competition between different types of meta effect photochemistry observed in acidic solutions containing water. The photochemical pathways of 3-hydroxymethyl-3'-fluorobenzophenone (1) and 3-fluoro-3'-methylbenzophenone (2) were explored in several solvents, including acidic water-containing solutions, using time-resolved spectroscopic experiments and density functional theory computations. It is observed that 1 can undergo a photoredox reaction and 2 can undergo a meta-methyl deprotonation reaction in acidic water-containing solutions. Comparison of these results to those previously reported for the analogous BP derivatives that contain only one substituent at a meta position indicates the introduction of electron-donating (such as hydroxyl) and electron-withdrawing groups (such as F) on the meta positions of BP can influence the meta effect photochemical reactions. It was found that involvement of an electron-donating moiety facilitates the meta effect photochemical reactions by stabilizing the crucial reactive biradical intermediate associated with the meta effect photochemical reactions.

Observation of excited state proton transfer reactions in 2-phenylphenol and 2-phenyl-1-naphthol and formation of quinone methide species

The excited state intramolecular proton transfer (ESIPT) reactions from a phenol (naphthol) to a carbon atom in the adjacent aromatic ring of 2-phenylphenol (1) and 2-phenyl-1-naphthol (4) are prototypical examples of intramolecular proton transfer not mediated by solvent molecules. Femtosecond time-resolved transient absorption (fs-TA) studies are conducted for the first time to directly probe the formation of quinone methide (QM) species generated from the ESIPT pathways of 1 and 4. Steady-state absorption experiments demonstrated 1 exists mainly in its non-deprotonated form in neat MeCN and in water-MeCN solutions. Observation of the phenolate form in water-containing solution (MeCN-H2O, 1 : 1, v : v) in fluorescence spectra demonstrates the occurrence of an ESPT reaction between 1 and the surrounding water molecules. In neat MeCN a transient species that absorbs around 520 nm was detected in fs-TA spectra and was assigned to the QM species formed by ESIPT to the 2'-position. This transient signal is strengthened in cyclohexane. In a water-MeCN solution, an additional transient species assigned to the QM species at the 4'-position of 1 was also detected that absorbs around 485 nm. Similar results for 4 were observed, with the absorbance of the transient species being more intense, which suggests there is more efficient production of the QM species from 4, consistent with quantum yields for deuterium exchange in the distal ring reported for these compounds.

Contribution of cytosine-containing cyclobutane dimers to DNA damage produced by photosensitized triplet-triplet energy transfer

Mutagenic cyclobutane pyrimidine dimers (CPDs) can be induced in DNA through either direct excitation or photosensitized triplet-triplet energy transfer (TTET). In the latter pathway, thymines are expected to receive the excitation energy from the photosensitizer and react with adjacent pyrimidines. By using state-of-the art analytical tools, we provide herein additional information on the formation of cytosine-containing CPDs. We thus determined the yield of all possible CPDs upon TTET in a series of natural DNAs with various base compositions. We show that the distribution of CPDs cannot be explained only by excitation of individual thymines. We propose that the mechanism for TTET involves at least dinucleotides as the minimal targets. The observation of the formation of cytosine-cytosine CPDs also suggests that additional pathways are involved in this photosensitized reaction.

Photoactive assemblies of organic compounds and biomolecules: drug–protein supramolecular systems

Modification of the drug excited state properties within proteins provides information on binding and may result in a different photoreactivity.

Time-resolved spectroscopic characterization of a novel photodecarboxylation reaction mediated by homolysis of a carbon α-bond in flurbiprofen

Flurbiprofen (Fp), a nonsteroidal anti-inflammatory drug (NSAID) currently in use for arthritis pain relief and in clinical trials for metastatic prostate cancer, can induce photosensitization and phototoxicity upon exposure to sunlight. The mechanisms responsible for Fp phototoxicity are poorly understood and deserve investigation. In this study, the photodecarboxylation reaction of Fp, which has been assumed to underpin its photoinduced side effects, was explored by femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopic techniques in pure acetonitrile (MeCN) solvent. Density functional theory (DFT) calculations were also performed to facilitate the assignments of transient species. The resonance Raman and DFT calculation results reveal that the neutral form of Fp was the predominant species present in MeCN. Analysis of the ultraviolet/visible absorption spectrum and results from TD-DFT calculations indicate that the second excited singlet (S2) can be excited by 266 nm light. Due to its intrinsic instability, S2 rapidly underwent internal conversion (IC) to decay to the lowest lying excited singlet (S1), which was observed in the fs-TA spectra at very early delay times. Intriguingly, three distinct pathways for S1 decay seem to coexist. Specifically, other than fluorescence emission back to the ground state and transformation to the lowest triplet state T1 through intersystem crossing (ISC), the homolysis of the carbon α-bond decarboxylation reaction proceeded simultaneously to give rise to two radical species, one being carboxyl and another being the residual, denoted as FpR. The coexistence of the triplet Fp (T1) and FpR species was verified by means of TR(3) spectra along with ns-TA spectra. As a consequence of its apparent high reactivity, the FpR intermediate was observed to undergo oxidation under oxygen-saturated conditions to yield another radical species, denoted as FOR, which subsequently underwent intramolecular hydrogen transfer (IHT) and dehydroxylation (DHO) to form a final product, which could react with the carboxyl from the decarboxylation reaction to generate a minor final product. TD-DFT and transient state (TS) calculations for predicting the absorption bands and activation energies of the transient species produced in the photodecarboxylation reaction have provided valuable mechanistic insights for the assignment of the intermediate species observed in the time-resolved spectroscopy experiments reported here. The results of the time-resolved spectroscopy experiments and DFT calculations were used to elucidate the reaction mechanisms and intermediates involved in the photochemistry of Fp.

Drug-Drug Interactions within Protein Cavities Probed by Triplet-Triplet Energy Transfer

A new direct and noninvasive methodology based on transient absorption spectroscopy has been developed to probe the feasibility of drug-drug interactions within a common protein binding site. The simultaneous presence of (R)-cinacalcet (CIN) and (S)-propranolol (PPN) within human or bovine α1-acid glycoproteins (AAGs) is revealed by detection of (3)CIN* as the only transient species after laser flash photolysis of CIN/PPN/AAG mixtures at 308 nm. This is the result of triplet-triplet energy transfer from (3)PPN* to CIN, which requires close contact between the two drugs within the same biological compartment. Similar results are obtained with nabumetone and CIN as donor/acceptor partners. This new methodology can, in principle, be extended to a variety of drug/drug/biomolecule combinations.

Intraprotein formation of a long wavelength absorbing complex and inhibition of excited-state deprotonation in a chiral hydroxybiphenyl

The two enantiomers of 2-(2-hydroxybiphenyl-4-yl)propanoic acid ((S)- and (R)-BPOH) have been selected as probes for human serum albumin (HSA). Photophysical characterization in the absence of protein led to emission maxima, singlet energies, quantum yields, and fluorescence lifetime values of 332 nm, 91 kcal mol(-1), and 0.28 and 1.8 ns for BPOH or 414 nm, 79 kcal mol(-1), and 0.31 and 3.3 ns for the corresponding phenolate BPO(-); the pK(a)* was found to be 1.17. In the presence of HSA, a light-absorbing ground-state complex (S)-BPOH@HSA was detected (maximum at ca. 300 nm) whose intensity increased with increasing protein concentration. The fluorescence spectra of (S)-BPOH in PBS, after addition of HSA, revealed a progressive diminution of the phenolate band, indicating that excited-state deprotonation is disfavored within the hydrophobic protein cavities. A similar trend was observed for (R)-BPOH, but the extent of deprotonation was significantly lower for this enantiomer. Addition of increasing amounts of the site II displacement probe (S)-ibuprofen ((S)-IBP) to BPOH@HSA led to a significant decrease of the absorption maximum at ca. 300 nm and to a recovery of the phenolate emission band at ca. 410 nm, which were again configuration dependent. The transient absorption spectrum of (S)-BPOH consisted on a broad band centered at 380 nm, attributed to the first triplet excited state. A dramatic enhancement of the triplet lifetimes within HSA was observed (19.0 μs within protein versus 1.3 μs in bulk PBS), although no stereodifferentiation was noticed in this case.

Photochemical properties and phototoxicity of Pazufloxacin: a stable and transient study

Photochemical properties and phototoxicity of Pazufloxacin (PAX) were systematically investigated in aqueous solutions using UV-Vis, fluorescence, laser flash photolysis, pulse radiolysis and SDS-PAGE gel electrophoresis techniques. PAX triplet-state ((3)PAX(*)) absorption spectra (λ(max)=570 nm) was determined. (3)PAX(*) was quenched by PAX and O(2), with rate constants of 6.9×10(8) and 3.2×10(8) dm(3) mol(-1) s(-1), respectively. The pK(a) values (5.7 and 8.6) for the protonation equilibrium were determined by UV-Vis and fluorescence techniques. The PAX triplet energy (E(T)=260.3 kJ/mol) was obtained using energy transfer method. The reaction of electron transfer from tryptophan (TrpH) and dGMP to (3)PAX(*) was found with rate constants of 8.8×10(7) and 8.7×10(6) dm(3) mol(-1) s(-1), respectively. The rate constants for reactions of ()OH, SO(4)(-) and hydrated electron with PAX were found to be 5.8×10(8), 2.1×10(9) and 9×10(9)d m(3) mol(-1) s(-1), respectively. Based on the results obtained, a rational scheme for dGMP, TrpH and lysozyme photodamage induced by PAX was proposed.

Triplet excited state behavior of naphthalene-based pseudopeptides in the presence of energy donors

In this work, the triplet state behavior of naphthalene-based pseudopeptides with amide-based macrocyclic or lateral chain substructures has been investigated in the presence of benzophenone and/or biphenyl, as suitable energy-donating chromophores. Their behavior has been compared with that of 1,4-dimethylnaphthalene as model compound. In all the cases, the triplet-triplet absorption of naphthalene is detected by transient absorption spectroscopy, upon selective excitation of benzophenone at 355 nm. The kinetics of formation and decay of this species is markedly slower in the pseudopeptides, due to retardation of triplet-triplet energy transfer and exciplex formation. Finally, the delayed fluorescence detected in the model naphthalene is absent in the pseudopeptides. The concept can, in principle, be exploited for the study of excited-state interactions in supramolecular systems.

Excited-state interactions in flurbiprofen-tryptophan dyads

Fluorescence and laser-flash photolysis measurements have been performed on two pairs of diastereomeric dyads that contain the nonsteroidal anti-inflammatory drug (S)- or (R)-flurbiprofen (FBP) and (S)-tryptophan (Trp), which is a relevant amino acid present in site I of human serum albumin. The fluorescence spectra were obtained when subjected to excitation at 266 nm, where approximately 60% of the light is absorbed by FBP and approximately 40% is absorbed by Trp; the most remarkable feature observed in all dyads was a dramatic fluorescence quenching, and the residual emission was assigned to the Trp chromophore. In addition, an exciplex emission was observed as a broad band between 380 and 500 nm, especially in the case of the (R,S) diastereomers. The fluorescence lifetimes (tauF) at lambdaem=340 nm were clearly shorter in the dyads than in Trp-derived model compounds; in contrast, the values of tauF at lambdaem=440 nm (exciplex) were much longer. On the other hand, the typical FBP triplet-triplet transient absorption spectrum was obtained when subjected to laser-flash photolysis, although the signals were less intense than when FBP was directly excited under the same conditions. The main photophysical events in FBP-Trp dyads can be summarized as follows: (1) most of the energy provided by the incident radiation at 266 nm reaches the excited singlet state of Trp (1Trp*), either via direct absorption by this chromophore or by singlet singlet energy transfer from excited FBP (1FBP*); (2) a minor, yet stereoselective deactivation of 1FBP* leads to detectable exciplexes and/or radical ion pairs; (3) the main process observed is intramolecular 1Trp* quenching; and (4) the first triplet excited-state of FBP can be populated by triplet-triplet energy transfer from excited Trp or by back-electron transfer within the charge-separated states.

The triplet energy of thymine in DNA

Norfloxacin (NFX) photosensitizes formation of thymine dimers (T<>T) in DNA, while its N(4') acetyl derivative (ANFX) does not. This is evident from the observation of single-strand breaks after enzymatic treatment with T4 endonuclease V and subsequent gel electrophoresis. The triplet energies of NFX and ANFX are estimated at 273 and 268 kJ/mol, respectively, on the basis of triplet-triplet energy transfer quenching by a set of biphenyl and naphthalene derivatives. Hence, the triplet energy of thymine in DNA (i.e., the value for a photosensitizer to produce T<>T) can be estimated at 270 kJ/mol.

The Long-Lived Triplet Excited State of an Elongated Ketoprofen Derivative and Its Interactions with Amino Acids and Nucleosides

The aim of the present work was to find a ketoprofen (KP) equivalent suitable for time-resolved studies on the interactions of its KP-like triplet state with biomolecules or their simple building blocks, under physiologically relevant conditions. Such a compound should fulfill the following requirements: (i) it should be soluble in aqueous media; (ii) its triplet lifetime should be longer than that of KP, ideally in the microsecond range; and (iii) its photodecarboxylation should be slow enough to avoid interference in the time-resolved studies associated with formation of photoproducts. Here, the glycine derivative of ketoprofen (KPGly) has been found to fulfill all the above requirements. In a first stage, the attention has been focused on the photophysical and photochemical properties of KPGly, and then on its excited-state interactions with key amino acids and nucleosides. In acetonitrile, the typical benzophenone-like triplet-triplet absorption (3KPGly) with lambda(max) at 520 nm and a lifetime of 5.3 micros was observed. This value is very close to that of 3KP (5.6 micros) obtained under the same conditions. In methanol, the 3KPGly features were also close to those of 3KP with detection of a short-lived triplet state that evolves to give a ketyl radical. By contrast with the behavior of KP, in deaerated aqueous solutions at pH = 7.4, the transient detected in the case of KPGly displayed two bands at lambda(max) at 330 and 520 nm, very similar to those observed in acetonitrile solution but with a lifetime of 7.5 micros at 520 nm. Hence, it was assigned to the KPGly triplet. In the case of KP, efficient decarboxylation occurs in the subnanosecond time scale, via intramolecular electron transfer. This process gives rise to a detectable carbanion intermediate (lifetime approximately 250 ns) and prevents detection of the shorter-lived 3KP signal. In a second stage, the attention has been focused on the excited-state interactions between 3KPGly and amino acids or nucleosides; for this purpose, 2'-deoxyguanosine (dGuo), thymidine (Thd), tryptophan (Trp), and tyrosine (Tyr) have been chosen as photosensitization targets. In general, efficient quenching (rate constant kq > 109 M(-1) x s(-1)) was observed; it was attributed for dGuo, Tyr, and Trp to a photochemical reaction involving initial electron transfer from the biological target to 3KPGly, followed by proton transfer from the amino acid or the nucleoside radical cation to KPGly-*. As a matter of fact, ketyl radical together with guanosinyl, tyrosinyl, or tryptophanyl radicals were detected; this supports the proposed mechanism. The results with Thd were somewhat different, as the efficient 3KPGly quenching was ascribed to oxetane formation by a Paterno Büchi photocycloaddition.

Triplet excited fluoroquinolones as mediators for thymine cyclobutane dimer formation in DNA

A series of fluoroquinolones (FQs), including enoxacin (ENX), pefloxacin (PFX), norfloxacin (NFX), its N(4')-acetyl derivative (ANFX), ofloxacin (OFX), and rufloxacin (RFX) have been investigated to determine their potential as DNA photosensitizers via thymine cyclobutane dimer (T<>T) formation in DNA. At fluoroquinolone concentrations and light doses insufficient to produce direct single strand breaks, ENX, PFX, and NFX were able to produce T<>T dimers in DNA, revealed by enzymatic treatment with T4 endonuclease V. By contrast, ANFX, OFX, and RFX were inefficient in this assay. The absolute values of the triplet energies of ENX, PFX, NFX, ANFX, OFX, and RFX were estimated by means of laser flash photolysis, using flurbiprofen, 4-biphenylcarboxylic acid, and naproxen as energy acceptors. They were found to be 273, 269, 269, 265, 262, and 253 kJ/mol, respectively. Other triplet excited state properties of the FQs, including quantum yields and lifetimes, were also studied. All the results indicate that the threshold ET value required for a given compound to become a potential DNA photosensitizer via T<>T formation is in the range defined by the triplet energies of NFX and ANFX (265-269 kJ/mol). This provides the basis for an alert rule: any chemical (drugs, cosmetics, pesticides, etc.) with higher ET has to be considered with regard to its potential photogenotoxicity.

Use of Triplet Excited States for the Study of Drug Binding to Human and Bovine Serum Albumins

The triplet excited states of (S)- and (R)-flurbiprofen (FBP) have been used as reporters for the microenvironments experienced within the binding sites of human and bovine serum albumins. Regression analysis of triplet decay provides valuable information on the degree of protection that these excited states are afforded from attack by a second FBP molecule, oxygen, or other reagents. The multiexponential fitting of these decays can be satisfactorily correlated with the distribution of the drug among the two binding sites and its presence as the noncomplexed form in the bulk solution. This assignment has been confirmed by using (S)-ibuprofen or capric acid as selective site II replacement probes. Triplet lifetimes and site occupancy are sensitive to the type of serum albumin employed (human versus bovine). Finally, the binding behaviour of (S)- and (R)-FBP exhibits little stereoselectivity.

Triplet excited States as chiral reporters for the binding of drugs to transport proteins

The triplet excited state of flurbiprofen methyl ester (FBPMe) has been used as a chiral reporter for the two binding sites of human serum albumin (HSA). The occupation level of the binding sites has been estimated from regression analysis of the triplet decays at several [FBPMe]/[HSA] ratios. The data agree with two high affinity binding sites (I and II) that are populated to a different extent. A remarkable stereodifferentiation has been found in the drug triplet lifetimes within the protein microenvironment.